1. Field of the Invention
This invention relates to printed wiring boards (PWB's) also referred to as printed circuit boards (PCB's) when populated with components. More particularly, it relates to printed wiring boards having differential pair signal traces.
2. Description of the Problem and the Related Art
In digital circuits, it is common to represent a binary “zero” with one voltage level and a binary “one” with another, different voltage level. By way of example, a logical “one” might be represented by a +5 volt signal level and a logical “zero” by 0 volts on a given signal line. Inasmuch as voltages are relative, it is common to reference the signal level to ground. Signals formed in this way are referred to as single-ended signals.
One disadvantage of this scheme is that the noise margin, or level between the guaranteed zero and one levels, is relatively small. It is preferable—i.e., more definite—to have a large noise margin. This is especially true at high frequencies where settling times can be relatively long due to ringing and other high frequency components of fast-rise signals.
The above-described disadvantage can be avoided by the use of differential pair signal transmission means. In this way, the signal state is not referenced from either ground or a high level but is based on the voltage difference between the two signals being above a given level. One representative case is shown in FIG. 1 wherein differential pair AB is comprised of Line A and Line B which may be at 0 or +5 vdc. A logical zero is represented by Line B being high—e.g., about +5 vdc—and line A being low—e.g., about 0 vdc. A logical one is represented by Line A being high and Line B being low. This scheme approximately doubles the noise margin as compared to single-ended cases.
Printed wiring boards (PWB's) or printed circuit boards (PCB's) may have signal lines comprised of traces on the top, bottom and/or interior layers of the board.
Signal wires (traces) on the printed wiring board carry signals between modules and between modules and input/output connections to the wiring board. In some circumstances, it is necessary for the signal lines to be routed on different layers in a PWB. “Vias” or via service terminals (VST's) are used to perform this layer-to-layer connection.
Vias may be “blind” holes which are open to one surface of the wiring board but do not extend to the opposite surface; they may be “buried” holes which connect only internal layers of the board and do not extend to either surface of the printed wiring board; or they may be plated through-holes which extend the full thickness of the board and open to solder pads on both surfaces of the board.
Conventional routing of differential pair signal lines on or within PWB's can result in bandwidth limitations for the signals due to impedance mismatches. This is particularly the case for high speed circuits—for example those running at 1 gigahertz and above. Any source of impedance mismatch becomes of greater concern as frequencies increase because of increased distortion of the waveforms in data transmission lines, resulting in poor eye patterns, jitters, overshoot, ringing, slow rise/fall time, etc. Furthermore, the impedance mismatch increases electromagnetic interference (EMI). Thus, any technique which reduces impedance mismatch for a reasonable cost is desirable.